max load

[blackfacespikeyhair]

My local DIY store sells 2.5mm2 cable which it says is suitable for up to 23 amps. Why is it then that it is normal to have a 30 amp fuse on a ring main?
Also, does anyone know how to put the "2" in "mm2" in superscript?

 

[ricicle]

The clue is in 'ring'.The theory behind it is the Ring Final Circuit is wired out from the CU, round the socket outlets and then returns to the CU.Then each leg should have an equal share of the 30/32A load ie 15/16A.That is why the ring tests are important as they ensure it is a ring and no one leg will carry a possible full 30/32A

 

[Taylortwocities]

Hold the Alt key down and type 0178 on yr numeric keypad. When you release the Alt key you will get a ²

There lots of these. If you've got MS Word go to Insert / Symbol. As you select various symbols from the table the shortcut is displayed at the btm of the window.

º ½ ¼ ³ etc etc

TTC

 

[blackfacespikeyhair]

Why are we limited to 30 amps then? Sounds like 46 would be ok. Also, is the load always split exactly equally in each direction around the ring, or does it depend on, say, how far around the ring the load is?

 

[Adam_151]

Why are we limited to 30 amps then? Sounds like 46 would be ok. Also, is the load always split exactly equally in each direction around the ring, or does it depend on, say, how far around the ring the load is?

Balence won't be spot on (if all the heavy loads are on one side of centre it'll be quite a way off) which is why we don't double, the old rules used to say cable rateing of 2/3 fuse rateing, but now they say 20A cable rateing for upto 32A fuse IIRC (I seem to remember a 'wireing matters' article about the ammendmant)

And by the way, it has a rateing of *upto* 27A not 23A (obviously depending on how it is installed)

 

[blackfacespikeyhair]

Thanks for all your answers by the way.

In response to Adam's comment, would it therefore be possible to overload the cable if there were, say, a 30 amp load very close to one end of the ring?

Incidentally, I got the figure of 23 from the cable packaging. I guess 27 is a peak rating whereas 23 is considered "normal use" perhaps??.

 

[Spark123]

A ring doesn't have an end as such, the ring leaves the CU on one leg and loops to each socket in turn before returning to the CU. We limit to 32A as all of the load may not be spread earound qually around the ring circuit, infact all 32A may be loaded on the first few sockets on one side of the ring. This then relies on ohms law to spread the current through the parallel paths of the cables. This is also one reason why some here don't really like the idea of ring circuits. A spur from a ring is allowed as long as it is only feeding one point such as a socket or fused spur unit whereas the overload protection is provided by the fuses in the plugtop/fsu and short circuit protection is provided by the 32A breaker. Here is a picture of a ring final socket circuit. Hope this makes sense.

 

[Adam_151]

The current in each leg is divided up in proportion to the leg length of a ring, if you put a 10A load in the middle, you'll get 5A down each side, move it to a quater way around and you get 7.5A and 2.5A, if its only a 10th of the way around you get 9A and 1A. You can't have a 30A load on a BS1363 plug obviously, but if you had 3 sockets clustered towards the end of the ring and had a 10A load on each then you could see the current divided up as something like 27A and 3A (if you had them a 10th of the way around), its almost as bad as a broken ring (where the ring accidentally becomes split somewhere along its length and is not a ring anymore)

When installing a ring, you should be careful to spead the sockets and loads out pretty evenly, no good wireing each one in turn, ending up 30m away from the fuse and putting a cable with no sockets on in to order to complete, much better to to get every second socket on the way out, and the remaining half on the way back

 

[dingbat]

Much better, all round, to install radial circuits, properly protected.